The present disclosure generally relates to optical recording media. In particular, it relates to a phase change optical recording medium including a plurality of stacked recording layers.
Phase change optical recording media are a type of optical recording media on which information can be recorded, read, and erased by irradiation with a laser beam. Information is recorded, read, and erased on a phase change optical recording medium by utilizing the crystal-amorphous phase change or the phase change between different crystal phases. Examples of the phase change optical recording media available in the market include rewritable compact disc (CD-RW), rewritable digital versatile disc (DVD-RW), digital versatile disc-random access memory (DVD-RAM), and Blu-ray disc. To meet recent demand for a higher storage capacity, media that include a stack of two or more recording layers (e.g., dual layer disc) has been introduced.
An example of the structure of a dual layer disc of a phase change optical recording medium type is as follows. A first recording layer is formed on a support substrate, a second recording layer is formed on the first recording layer with an intermediate layer transparent to the read/write wavelength therebetween, and a light-transmitting protective layer transparent to the read/write wavelength is disposed on the second recording layer. The laser beam used in recording and reading is incident on an optical recording medium through an object lens on the light-transmitting protective layer side. The laser beam passing through the object lens is focused on a first recording layer or a second recording layer to record and read information.
The feature of a dual layer disc is that the second recording layer is configured as a semi-transmissive recording layer having an ability to transmit light so that reading and writing can be conducted on the first recording layer. The second recording layer is made by stacking a dielectric material, a metal, a phase change recording material, etc., so that read and write functions can be performed. A typical stack structure include a first dielectric layer, a metal reflective layer, a second dielectric layer, a phase change recording material layer, and a third dielectric layer that are stacked in that order from the substrate side. A light transmittance of about 45% to 55% is imparted so that the read/write power and the reflectance are the same between the first recording layer and the second recording layer when viewed from an optical disc read/write device (drive).
In the second recording layer (semi-transmissive recording layer) having such a configuration, the second dielectric layer between the metal reflective layer and the phase change recording material layer has a function of increasing the absorption efficiency of the recording layer by adjusting the optical distance and a function of increasing the signal amplitude by increasing the change in amount of reflected light between before and after recording.
An oxide, a nitride, a sulfide, a carbide, a fluoride, carbon, etc., are used alone or in combination as a mixture as the material constituting this second dielectric layer. The second dielectric layer may have a stack structure combining these materials. In particular, International Publication WO2008/018225 proposes that the interface layer on the recording layer side contain an oxide such as indium oxide (In2O3), chromium oxide (Cr2O3), or gallium oxide (Ga2O3), and silicon.